feat: Mitmachen-Formular nativ (löst OpnForm ab)

- MitmachenFormComponent: Name der BI*, Anliegen*, Ansprechpartner,
  Telefon, E-Mail, Datei-Upload (max 5×5MB, JPG/PNG/WebP/GIF/PDF),
  Consent — Honeypots + Client-Age + Turnstile wie Kontaktformular
- /api/mitmachen: Multipart-Validierung (shared Modul), Forwarding an
  CMS /api/forms/mitmachen/submit-multipart
- internal_component: form-mitmachen registriert
- ~35 neue mitmachen_*-Translation-Keys

Prod braucht BODY_SIZE_LIMIT=30M (adapter-node default 512K) — via
secrets .env.prod deployt.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/block-types.ts

@@ -469,7 +469,7 @@ export type CalendarBlockData = Omit<
 };
 
 /** Internal component block (_type: "internal_component"). */
-export type InternalComponentId = "form-contact";
+export type InternalComponentId = "form-contact" | "form-mitmachen";
 
 export interface InternalComponentBlockData {
   _type?: "internal_component";